Why does the prototyping day of the Design Sprint suck for the team?

OK, we just ran our first (Google Ventures) Design Sprint according to Jake Knapp's et. al. "Sprint" book. It was a very productive and overall pleasant event.

Putting on my meta UX hat, I handed out questionnaires at the end of every day to gather feedback for the individual activities conducted and the overall approval of the results that the team achieved that day.

We were to re-envision a very old school Windows Desktop B2B application. Everything was honky dory, until we reached the prototyping day. We went into the day with a detailed story board and a clear vision of what had to be build.

The product should look like a native Windows Office application. Hence, we followed the book's recommendation to use PowerPoint. To speed up things, we bought a set of templates from Keynotopia for not having to build windows, buttons and the like ourselves.

However, it wasn't too much fun, after all. These were my team's responses on a 5-point Likert scale (0 = Don't agree at all; 4 = Fully agree]:

• I like the resulting prototype: mean 1.7, median 2
• PowerPoint was the right choice as prototyping tool: mean 0.7, median 1
• Keynotopia templates were a time-saver: mean 1.3, median 1

From my observation, one major problem was the limited ability to split up remaining work once the stitcher started to put things together, and missing features like layers and symbols like one has in Illustrator.

"Stitcher" is a term from the Sprint book. It refers to the person who gathers individual screens and components from the "Makers", who create these parts. Her task is to iron out inconsistencies and stitch the screens together to form the complete prototype.

The only thing that prevented the Sprint from falling apart was the fact that prototyping happened on Friday and User Tests were scheduled for Monday. So some weekend-heroics yielded a polished prototype which actually outperformed our existing software significantly (in the statistical sense, 5 users filled out a SUS-questionnaire each for the old software and the PowerPoint-prototype).

So in the end the Sprint was a full success. However, I'd rather not expose my team to a set of tools that let them down before.

Has anyone encountered a situation like this before? Is it a tooling problem? Is it a coordination problem?

Edit: After fixing the prototype on the weekend the result was rated 3.7 on the [0..4]-scale, and all team-members would wholeheartedly recommend the Design Sprint to a friend with a design problem.